HS2 was forced to temporarily halt excavation work on West Ruislip Tunnel on February 18, following reports of bubbling foam emerging on the surface above.
The area on a rugby pitch in Ruislip was cordoned off and tunnelling paused for approximately two hours, amid fears from local residents that a sinkhole had appeared.
An HS2 spokesman said that investigations are ongoing, although it is believed that a Tunnel Boring Machine went through a small pre-existing borehole. This forced slurry to travel up the borehole and pool on the surface.
Once tunnelling had resumed and the TBM passed by, the borehole was subsequently sealed behind one of HS2’s TBM tunnel rings, and the foam cleared up.
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